The present invention relates to apparatus and a process for the liquid granulation of slag. More particularly, this invention relates to the granulation of blast furnace slag, in which the slag in a molten state is caused to drop into a granulation enclosure into which water is injected under pressure in the form of a number of superimposed currents and from which the slurry consisting of granulated slag and water is evacuated to apparatus for filtration and separation of the water from the granulated slag. The filtration and separation apparatus comprises a rotary filter drum which rotates about a horizontal axis and which is fitted internally with filtration buckets which during the rotation of the drum are immersed in the slurry and carry the granulated slag upwards in order to discharge it onto an evacuation belt.
In the field of slag granulation, as in others, efforts are being made more than ever before to economize in energy and in raw materials. European patent application No. 0043605 and U.S. patent application Ser. No. 267,880, now U.S. Pat. No. 4,374,645 already made it possible to achieve considerable progress in this direction resulting from the fact that it proposes a separation of the two essential functions of the water, i.e., the granulation and the evacuation of the slag, for which purpose it provides that the water is to be injected into the granulation enclosure in the form of two superimposed and independent currents. This design makes it possible, in particular, to keep the two currents at different pressures and temperatures from each other.
However, even if the teachings of the above-identified European patent application enable the consumption of water to be noticeably reduced, the total output nevertheless remains constant and is selected in such a way as to be sufficient for the granulation of a maximum output of slag. However, the output of molten slag from a blast furnace, for example, does not remain constant over a period of time; and the maximum output may reach twice the average value of slag output. Thus, excess water is consumed whenever slag output is less than maximum. Up to the present time, unfortunately, it has not been possible or desirable to adapt the output of granulation water to the output of molten slag, for a number of important reasons.
One important reason is that in most cases, even if the delivery of slag is found to vary, these variations in the slag output cannot be exactly measured.
Another important reason is that if it is desired to modify the volume of water passing through a given orifice, the water pressure has to be modified; and this pressure modification has to be fairly considerable, since the pressure increases in proportion to the square of the volume. This means that if the output is increased, starting from a certain average value, a high increase in the pressure is required, so that the apparent saving of granulation water is more than nullified by the additional energy required for the operation of the pumps. Similarly, if the output is reduced in relation to a certain average, the pressure is no longer sufficient to maintain the granulation process and the evacuation of the slag.
On the other hand, if the rate of flow of granulation water is kept at a value corresponding to an average rate of flow of the slag to be granulated the slag will be insufficiently granulated and cooled when the output of slag is increased. Consequently, in order to ensure adequate granulation and cooling of the slag in all cases, the rate of flow of the current or currents of water in the prior art is selected in such a way that it will be sufficient to enable the slag, even when the output thereof is at its maximum, to be correctly granulated and cooled.
The obvious drawback of the prior art systems is that whenever the output of molten slag is not at its maximum an excessive quantity of granulation water is injected. This means not only waste but a more complicated final filtration, since the water once again has to be separated from the granulated slag if the latter is to be usable. At the same time a waste of energy occurs, since the pumps have to be powered in order to supply volumes of granulation water which would normally not be necessary. These considerations also apply to all the pumps involved in the complete granulation circuit (recirculation, decantation, cooling, etc.).
Since there was heretofore no means of regulating the rate of flow of granulation water in accordance with that of the slag, or the technical complications involved were out of proportion to the savings obtainable, the aforementioned difficulties resulting from the maintenance of the maximum flow of granulation water were reluctantly accepted.